Numerous patents and publication have been issued on the fabrication of semiconductor nanoparticles (also called “quantum dots” or “nanocrystals”) and their applications in the last few years. These semiconductor nanoparticles are used as a lasing medium in a laser, as fluorescent tags in biological testing methods and as electronic devices.
A relatively new correlative method for easier manipulation and spatial organization of the nanoparticles has been proposed in which the nanoparticles are encapsulated in a shell. The shells which encapsulate the nanoparticles are composed of various organic materials such as Polyvinyl Alcohol (PVA), PMMA, and PPV. Furthermore, semiconductor shells have also been suggested.
For example, U.S. Pat. Nos. 6,225,198 and 5,505,928, incorporated herein by reference, disclose a method of forming nanoparticles using an organic surfactant. The process described in the U.S. Pat. No. 6,225,198 patent includes providing organic compounds, which are precursors of Group II and Group VI elements, in an organic solvent. A hot organic surfactant mixture is added to the precursor solution. The addition of the hot organic surfactant mixture causes precipitation of the II-VI semiconductor nanoparticles. The surfactants coat the nanoparticles to control the size of the nanoparticles. However, this method is disadvantageous because it involves the use of a high temperature (above 200° C.) process and toxic reactants and surfactants. The resulting nanoparticles are coated with a layer of an organic surfactant and some surfactant is incorporated into the semiconductor nanoparticles. The organic surfactant negatively affects the optical and electrical properties of the nanoparticles.
In another prior art method, II-VI semiconductor nanoparticles were encapsulated in a shell comprising a different II-VI semiconductor material, as described in U.S. Pat. No. 6,207,229, incorporated herein by reference. However, the shell also interferes with the optical and electrical properties of the nanoparticles, decreasing quantum efficiency of the radiation and the production yield of the nanoparticles.
The commercialization of the nanoparticles has also been hampered due to the high cost of production of the nanoparticles. The methods used for synthesis are extremely toxic at high temperatures and hence pose significant safety problems during mass production.
Furthermore, it has been difficult to form nanoparticles of a uniform size. Some researchers claimed to have formed nanoparticles in a solution having a uniform size based on transmission electron microscopy (TEM) measurements and based on approximating nanoparticle size from the position of the exciton peak in the absorption spectra of the nanoparticles. However, the present inventor has determined that both of these methods do not lead to an accurate determination of nanoparticle size in the solution.
TEM allows actual observation of a few nanoparticles precipitated on a substrate from a solution. However, since very few nanoparticles are observed during each test, the nanoparticle size varies greatly between observations of different nanoparticles from the same solution. Therefore, even if a single TEM measurement shows a few nanoparticles of a uniform size, this does not correlate to an entire solution of nanoparticles of a uniform size.
Using the absorption spectra exciton peak position to approximate nanoparticle size is problematic for a different reason. The exciton peak position does not show whether the individual nanoparticles in a solution are agglomerated into a large cluster. Thus, the size of the individual nanoparticles that is estimated from the location of the exciton peak in the absorption spectra does not take into account that the individual nanoparticles have agglomerated into clusters.
For example, Evident Technologies (www.evidenttech.com) markets EviDots® CdSe nanocrystal test kits. The Evident Technologies website indicates that the nanocrystals in these kits are coated with a surfactant and can be stored unopened in toluene solvent for up to two months. This website also implies that the nanoparticles in the test kit solution have a uniform size. It appears that the nanoparticle size was approximated from the exciton peak position of the absorption spectra of these nanoparticles. However, when the present inventor arranged for the size of these EviDots® nanoparticles to be measured by a photon correlated spectroscopy (PCS) method, the results indicated that the nanoparticles in the solution have agglomerated into large clusters and have a non-uniform size distribution. Thus, while the agglomerated nanoparticles have acceptable optical properties, they have unacceptable mechanical properties due to the agglomeration for uses which require a precise size distribution, such as for biological marker use. Furthermore, the EviDots® nanoparticles agglomerate into large, visible clumps and precipitate out of the solution onto the bottom of the vial in less than an hour after the toluene containing vial is unsealed.